Power MOSFETs have typically been developed for applications requiring power switching and power amplification. For power switching applications, the commercially available devices are typically double diffused MOSFETs (DMOSFETs). In a typical transistor, much of the breakdown voltage BV is supported by a drift region, which is lowly doped in order to provide a higher breakdown voltage BV. However, the lowly doped drift region also produces high on-resistance Rds-on. For a typical transistor, Rds-on is proportional to BV2.5. Rds-on therefore increases dramatically with increase in breakdown voltage BV for a conventional transistor.
Superjunctions are a well known type of semiconductor device. Superjunction transistors provide a way to achieve low on-resistance (Rds-on), while maintaining a high off-state breakdown voltage (BV). Superjunction devices include alternating P-type and N-type doped columns formed in the drift region. In the OFF-state of the MOSFET, the columns completely deplete at relatively low voltage and thus can sustain a high breakdown voltage (the columns deplete laterally, so that the entire p and n columns are depleted). For a superjunction, the on-resistance Rds-on increases in direct proportion to the breakdown voltage BV, which is a much less dramatic increase than in the conventional semiconductor structure. A superjunction device may therefore have significantly lower Rds-on than a conventional MOSFET device for the same high breakdown voltage (BV) (or conversely may have a significantly higher BV than a conventional MOSFET for a given Rds-on).
Unclamped Inductive Switching (UIS) is a context sensitive term used to describe a Power MOSFET's ability to sustain current in the avalanche mode (a condition when the drain-source voltage exceeds the bulk break down of the Power MOSFET) without permanent damage. The UIS is a figure commonly used to measure the robustness of a MOSFET. One of the key points to achieving a high UIS is ensuring that the termination regions of the MOSFET do not break down before the active cell regions do. The active cell regions cover much larger area than the termination region and so can tolerate a much larger avalanche current than the termination regions. In conventional superjunction devices, unstable UIS caused by process variation or lower termination or corner region BV may exist. Furthermore, when many superjunction devices are formed in a common wafer, Rds-on and BV may vary across due to process variation across the wafer.
It is within this context that embodiments of the present invention arise.